Magnetic track bank cards are already used for this purpose on a wide scale. The first steps have been taken for changing over to bank cards including microcircuits or chips. In cards of this type, the memory processes a sum of money like a number which is encoded on a plurality of bytes or memory cells, in the same way as an accounting operation or a check are processed. There is thus no simple relationship between each memory bit and a unit of monetary value. It is therefore necessary to take steps to ensure that the card user and the card itself are credit-worthy. The means most commonly used for this purpose is the confidential code. Verifying a confidential code requires a procedure which complicates card manipulation.
To some extent, systematic use of a confidential code can be omitted by using so-called "prepaid" cards. In such cards, each memory bit possesses a monetary value fixed in advance. As the name specifies, the card owner pays for the value thereof prior to making use of it. The card itself is thus totally credit-worthy. And since the sums of money concerned are relatively small, there is no need to verify that a card holder is indeed its legitimate owner.
Prepayment type "telephone" cards are currently in use in France with public payphones. They are easy to implement since all telephone charges can be defined as a number of telephone units to be paid.
There is a need for prepaid type cards which are suitable for a wider range of applications than telephone cards. In other words there is a need for an electronic purse card or wallet card.
One of the problems encountered is that of optimizing the card, given that its memory capacity is limited and that the number of times a card can be reloaded is also limited. Other factors are involved, such as the fact that the memory is made in erasable programmable read-only memory (EPROM) technology, or in electrically-erasable programmable read-only memory (EEPROM) technology, also written "E.sup.2 PROM". A distinction should also be made between cards containing a memory associated with a hard-wired logic circuit, generally on the same chip, and cards containing a memory and a microprocessor either on a single chip or else on two separate chips.
Another point to observe is that it is necessary, in practice, to avoid the need for "giving change" during transactions. This means that a card needs reloading only when the user desires to obtain more cash (in the card), which operation may require verification of the user's credit-worthiness.
An object of the present invention is to provide solutions to these problems.